DVD-70C The Seven Sins of Lead Free Soldering - IPC Training ...

DVD-70C ScriptSin number 2 – Mishandling Moisture Sensitive DevicesOne of the more dastardly sins in lead free assembly is the mishandling of moisture sensitivedevices. These components are at even greater risk because of the higher soldering temperaturesused during the lead free process.When we’re in the midst of our busy day, we may not take the time to check the humidityindicator card after removing components from moisture barrier bags. Or we may forget to addfresh desiccant and to reseal the bags in a timely manner.In addition, it may slip our mind to examine the moisture sensitive label that specifies moistureclassification, storage time and floor life. We may also not be aware when the floor life of MSDshas been exceeded. Rather than properly baking the components that have exceeded time limits,we allow devices with excessive moisture to be reflow soldered. The combination of highsoldering temperatures and moisture inside the device creates the probability of componentfailure.Let’s quickly review what determines whether a component is moisture sensitive – and why theaccumulation of moisture can be a problem. Moisture sensitive means there is not a perfect sealbetween the internal circuitry and the package that surrounds it. These types of components arenot hermetically sealed like a ceramic component would be.Many common surface mount plastic packages are at risk – including small outline integratedcircuits, or SOICs; quad flat packs, or QFPs; plastic leaded chip carriers, or PLCCs; and ball gridarrays, or BGAs. The more complex and intricate the inside of the component is, the moresensitive the component becomes. When these types of packages are exposed to a typical factoryenvironment, moisture can accumulate inside the package.The reason moisture accumulation is a problem is the vapor pressure of the moisture inside thepackage greatly increases when the device is exposed to high temperatures during reflowsoldering operations.The exposure to these high temperatures and increased internal vapor pressures can result in thepackage cracking or the delamination of internal interfaces within the package. This is usuallycalled popcorning.In order to control and prevent problems from occurring during surface mount assembly or anytype of hot air rework, it’s important to do the following. Moisture sensitive devices need to bestored in moisture barrier bags, along with active desiccant and a humidity indicator card.When the humidity indicator card reveals that the allowed relative humidity has been exceeded,the components will need to be baked per the time periods specified in J-STD-033 – or yourcompany’s standard operating procedure.Carrier materials that components are stored in – such as trays, tubes and reels – can also affectthe moisture level in the moisture barrier bag. If these materials are not baked, extra desiccantwill need to be added to the moisture barrier bag.Exposing moisture sensitive components to the factory environment should be kept to a minimumduring the assembly process. Floor life often becomes an issue for MSDs when another hot job isneeded – and reels, trays and tubes have to be switched out to run that new job. If the exposure tothe factory environment exceeds the specified time, then the MSDs will need to go through a3